1. Field of the Invention
This invention relates to overcurrent trip units for electric circuit interrupting devices, and more particularly to trip units which provide multiple protection functions having a dependency relationship between selectable trip parameters, and which have a graphical representation of the current/time trip characteristic of the device displayed on an operator's panel.
2. Background Information
Circuit interrupters for industrial and commercial applications commonly provide multiple protection functions. For instance, such circuit interrupters typically provide instantaneous and long delay protection and may also provide short delay protection and/or ground fault protection. The instantaneous trip function protects the electrical conductors to which the current interrupting device is connected from damaging overcurrent conditions, such as short circuits. The long delay trip function protects the load served by the protected electrical system from overcurrents. The short delay trip function can be used to coordinate tripping of down stream circuit interrupters in a hierarchy of circuit interrupters. As implied, the ground fault trip function protects the electrical system from faults to ground.
Each circuit interrupter is designed for a specific maximum continuous current. This current rating may be set by selection of a resistor which converts the current to a voltage for use by the trip unit. In some cases, a single circuit interrupter frame may be easily adapted for installations which call for a range of maximum continuous currents, up to the design limits of the frame, through use of a removable rating plug by which the current rating of the device can be established.
Traditionally, the pick-up currents for the various protection functions have been selectable multiples or fractions of this current rating. Thus, instantaneous protection trips the device anytime the current reaches a selected multiple of the rated current, such as for example, ten times the rated current. Pick-up for short delay protection is a lesser multiple of the rated current, while pick-up current for long delay protection may be a fraction of the rated current.
The long delay and short delay trip functions require that the overcurrent condition exist for a period of time. Typically, the long delay trip function has been selected such that a trip signal is generated if the current exceeds six times the rated current for the selected long delay interval. As damage can also occur at lesser current levels for longer periods of time, an inverse time function is used for long delay protection. Thus, the smaller the current the longer the time to trip, and the larger the current, the shorter the time to trip. Typically, an I.sup.2 t characteristic has been used for long delay protection, although in some circuit breakers the inverse time function is programmable, and may include other functions such as It or I.sup.4 t. Such a circuit breaker with a programmable inverse time trip function is disclosed in commonly owned U.S. patent application Ser. No. 07/907,131 filed on Jun. 30, 1992. Typically, the short delay trip is only generated when the short delay pick-up current is exceeded for the short delay interval, although in some applications an inverse time function is used for short delay protection also.
If the current/time characteristic of a circuit interrupter is plotted on a logarithmic scale with current on the abscissa and time on the ordinate, the pick-up currents appear as vertical line segments, and the I.sup.2 t characteristic of the long delay, and if used for the short delay, appear as straight diagonal lines. The assignee of this application manufacturers circuit breakers in which a graphic representation of this response characteristic is provided on the operator's panel together with switches for setting the parameters of the protection functions located adjacent the appropriate section of the current/time characteristic. Such a circuit breaker is disclosed in U.S. Pat. No. 4,752,853.
With the traditional approach of having the instantaneous, short delay and long delay pick-up currents selectable as factors of the rated current, the protection functions are independent of one another, so that setting the parameters for one function does not affect the setting of parameters for another function. However, it has been found desirable in some applications to adjust the short delay trip function in coordination with the long delay trip function. Thus, trip units have been designed in which the short delay pick-up current is changed to a multiple of the long delay pick-up current, and not the rated current. This results in dependence of the short delay protection upon the long delay pick-up level.
There is a need to communicate this dependence of the short delay upon the long delay pick-up level and the adjustment of the inverse time function of the circuit interrupting device to the user.